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United States Patent |
5,148,963
|
Hicks
|
September 22, 1992
|
Method for solder nozzle height sensing
Abstract
The method of the present invention permit the accurate sensing of the
height of a solder nozzle above a circuit board during operation of the
solder nozzle. A non-contact limit switch, which provides an indication of
the presence of a planar surface at a preselected proximity, is mounted in
a fixed relationship with a solder nozzle which is manipulatable by means
of a robotic arm. A reference surface is provided which includes a
proximity sensor, such as a through beam fiber optic switch. The proximity
sensor is mounted a predetermined distance above the reference surface and
is utilized to detect the solder nozzle as it is moved toward the
reference surface. By noting the position coordinates of the robotic arm
at which the proximity sensor indicates the presence of the solder nozzle
at the predetermined distance above the reference surface, and the
position of the robotic arm at which the non-contact limit switch closes
in response to the proximity of the reference surface, it is possible to
accurately calculate a calibration offset value which may be utilized in
conjunction with the output of the non-contact limit switch to precisely
position the solder nozzle at a desired distance above a circuit board. In
one preferred embodiment of the present invention, the solder nozzle is
repeatedly moved toward the reference surface during calibration to ensure
that the calibration offset value obtained is statistically significant.
Inventors:
|
Hicks; Christopher A. (Austin, TX)
|
Assignee:
|
International Business Machines Corporation (Armonk, NY)
|
Appl. No.:
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879433 |
Filed:
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May 6, 1992 |
Current U.S. Class: |
228/102; 427/8 |
Intern'l Class: |
B23K 003/08 |
Field of Search: |
118/712,669
73/1 J
228/102
427/8
|
References Cited
U.S. Patent Documents
3074264 | Jan., 1963 | Polk | 73/37.
|
3502969 | Mar., 1970 | Courtney-Pratt et al. | 73/37.
|
3809308 | May., 1974 | Roeder et al. | 228/9.
|
4425061 | Jan., 1984 | Kindl et al. | 408/16.
|
4661368 | Apr., 1987 | Rohde et al. | 118/669.
|
4745557 | May., 1988 | Pekar et al. | 318/640.
|
4762578 | Aug., 1988 | Burgin et al. | 118/712.
|
4898117 | Feb., 1990 | Lederman et al. | 118/665.
|
4914460 | Apr., 1990 | Caimi et al. | 901/47.
|
4969107 | Nov., 1990 | Mizutani | 901/47.
|
Primary Examiner: Ramsey; Kenneth J.
Attorney, Agent or Firm: Dillon; Andrew J.
Parent Case Text
This application is a division of application Ser. No. 07/586,653, filed
Sept. 24, 1990, now U.S. Pat. No. 5,119,759.
Claims
I claim:
1. A method for controlling the height of a solder nozzle above a planar
member in a solder deposition system having a solder nozzle mounted to a
manipulatable robotic arm and adapted to dispense heated solder onto the
surface of said planar member, said method comprising the steps of:
mounting a non-contact limit switch in a fixed relationship with respect to
said solder nozzle, said non-contact limit switch generating a first
signal in response to the presence of a planar member within a preselected
proximity thereof;
providing a reference surface having a proximity sensor a predetermined
distance above said reference surface adapted to generate a second signal
in response to the presence of said solder nozzle at said predetermined
distance above said reference surface;
manipulating said solder nozzle toward said reference surface; and
utilizing said first signal, said second signal and said predetermined
distance to calculate a calibration offset value to be utilized to correct
the apparent height of said solder nozzle above said planar member wherein
variations in said fixed relationship between said non-contact limit
switch and said solder nozzle may be accommodated.
2. The method for controlling the height of a solder nozzle above a planar
member in a solder deposition system according to claim 1, further
including the step of raising said solder nozzle to actual operating
temperature prior to calculating said calibration offset value.
3. The method for controlling the height of a tool above a planar member
according to claim 2, further including the step of repeatedly
manipulating said tool toward said reference surface and utilizing average
values of said first signal and said second signal with said predetermined
distance to calculate said calibration offset value.
4. The method for controlling the height of a tool above a planar member
according to claim 3, further including the step of calculating a standard
deviation for said average value of said first signal and said second
signal to determine whether said average values are statistically
significant prior to calculating said calibration offset value.
5. The method for controlling the height of a solder nozzle above a planar
member in a solder deposition system according to claim 1, wherein said
step of manipulating said solder nozzle toward said reference surface
comprises the step of manipulating said solder nozzle toward said
reference surface at a speed substantially equal to actual solder
deposition speed.
6. The method for controlling the height of a solder nozzle above a planar
member according to claim 1, further including the steps of repeatedly
manipulating said solder nozzle toward said reference surface and
utilizing average values of said first signal and said second signal with
said predetermined distance to calculate said calibration offset value.
7. The method for controlling the height of a solder nozzle above a planar
member in a solder deposition system according to claim 6, further
including the step of calculating a standard deviation for said average
value of said first signal and said second signal to determine whether
said average values are statistically significant prior to calculating
said calibration offset value.
8. A method for controlling the height of a tool above a planar member in a
system having a tool mounted to a manipulatable robotic arm and adapted to
be urged toward said planar member, said method comprising the steps of:
mounting a non-contact limit switch in a fixed relationship with respect to
said tool, said non-contact limit switch generating a first signal in
response to the presence of a planar member within a preselected proximity
thereof;
providing a reference surface having a proximity sensor fixedly mounted a
predetermined distance above said reference surface adapted to generate a
second signal in response to the presence of said tool at said
predetermined distance above said reference surface;
manipulating said tool toward said reference surface; and
utilizing said first signal, said second signal and said predetermined
distance to calculate a calibration offset value to be utilized to correct
the apparent height of said tool above said planar member.
9. The method for controlling the height of a tool above a planar member
according to claim 8, wherein said step of manipulating said tool toward
said reference surface comprises the step of manipulating said tool toward
said reference surface at a speed substantially equal to actual tool
operation speed.
Description
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates in general to the field of automated solder
deposition systems and in particular to an improved method for depositing
solder onto solder wettable contact pads on a circuit board. Still more
particularly, the present invention relates to a method for accurately
maintaining a tool at a desired height above a circuit board or other
planar member.
2. Description of the Related Art
Solder distribution onto mounting pads for surface mount boards has
generally been accomplished in the semiconductor industry utilizing a
screening process. In a screen process artwork and screens must be
fabricated having the solder deposition pattern. Then, a precision
alignment process is carried out wherein the solder is screened onto the
surface mount pads. The solder parts used for this process requires a
substantially long cure and bake time. Thus, in addition to the necessary
complexity of the alignment process, this prior art technique is
relatively time consuming.
The prior art solder distributes technique utilizing screening is further
complicated by a requirement that the pattern mix very fine lead pitch and
width surface mound pads along with standard surface mount pads. For
example, in the case of tape automated bonding, the pitches vary from four
to twenty mils, while standard surface mount parts have pitches in the
range of twenty to fifty mils. Thus, this process required that different
amounts of solder be distributed on various parts of the board. Given the
precision required, it is common to utilize separate screening steps, one
for very fine lead pitch and width surface mount parts and a second for
the standard surface mount parts. There is also a possibility of damaging
the solder deposited in a previous step when multiple screening operations
are carried out. Additionally, screening fine line solder presents a
problem because the solder paste tends to stick in the openings of the
screen, as the openings get progressively narrower.
The prior art also discloses other techniques for depositing solder across
the surface of a printed circuit. Dip soldering and wave soldering are
both techniques which are known in the prior art. Wave soldering involves
pumping a molten molder through a nozzle to form a standard wave. In this
presence, the entire side of an assembly containing printed conductors
with the leads from the circuit components projecting through various
points generally travels at a predetermined rate of speed over the
standard surface of the wave of molten solder. The lower surface of the
assemblies is placed into contact with the upper fluid surface of the
wave.
By this technique, the solder wave in the first instance wets the joining
surfaces and promotes through hole penetration. This in turn helps to
assure the formation of reliable solder joints and fillets. Wave soldering
is illustrated in U.S. Pat. Nos. 3,705,457 and 4,360,144. An example of an
immersion technique is illustrated in U.S. Pat. No. 4,608,941 wherein
panels are immersed in a liquid solder bath and then conveyed to an air
knife which levels the molten solder on the panels. The air knife is
therefore used to effectively clear the panels of excess solder and only
the printed patterns retain the solder.
Another example of a solder leveler is contained in U.S. Pat. No.
4,619,841. The technique disclosed therein is used in conjunction with dip
soldering techniques. Other techniques of selective deposition of solder
onto printed circuit patterns are described in U.S. Pat. Nos. 4,206,254,
4,389,711 and 4,493,856.
U.S. Pat. No. 3,661,638 is also directed to a system for leveling and
controlling the thickness of a conductive material on the walls of
through-holes of a printed circuit board. That technique for removing the
excess amount of conductive material employs heating to melt a conductive
material after it has been deposited. Then, while the conductive material
is in a plastic state, gyrating the board to cause the plastic material to
move circumferentially about the throughhole and flow axially through the
through-hole.
More recently, several techniques have been proposed which utilizes a
solder nozzle which deposits solder onto solder wettable contact pads in
substantially uniform amounts on each pad. The tool utilized with such a
nozzle generally comprises a solder reservoir or plenum, a heating element
to melt the solder, and at the bottom of the reservoir, a foot which
contains the nozzle and which passes over the contact pads to be wetted
with solder. Examples of these types of systems are disclosed in U.S. Pat.
No. 4,898,112, filed Apr. 15, 1988, entitled "Solder Deposition System"
and U.S. Pat. No. 5,042,708, filed of even date herewith entitled "Solder
Placement Nozzle Assembly."
Methods utilized to adjust the height of a solder nozzle, such as those
described in the abovereferenced patent applications, are completely
manual and typically utilize mechanical shims or feeler gauges of known
thicknesses to measure the height between the tip of the solder nozzle and
the circuit board or planar member upon which solder is to be deposited.
Simple trial and error techniques were then utilized to adjust the nozzle
height until it was within a desired range. This technique is quite time
consuming and expensive.
A problem which exists with such systems is that the utilization of tools
which contact the soldering nozzle and the surface to be soldered
generally cause flux to be forced out from between the solder nozzle and
the surface to be soldered, resulting in flux covered set up tools and the
spreading of flux onto the soldering nozzle and the surface being
soldered. Another problem which exits with such systems is that the
surface being soldered varies in thicknesses and flatness and therefore
tool height needs to be measured at each solder application point.
The environment near a solder nozzle is generally filled with flux, flux
vapor, molten solder, solder oxides and solder vapor and as a result
attempts at utilizing contact sensors to ensure proper solder nozzle
height have been generally unsuccessful.
It should therefore be apparent that a need exists for a method and
apparatus which permits the accurate and efficient adjustment of a solder
nozzle height in a solder deposition system.
SUMMARY OF THE INVENTION
It is therefore one object of the present invention to provide an improved
automated solder deposition system.
It is another object of the present invention to provide an improved method
and apparatus for depositing solder onto solder wettable contact pads on a
circuit board.
It is yet another object of the present invention to provide an improved
method and apparatus for accurately maintaining a solder nozzle at a
desired height above a circuit board.
The foregoing objects are achieved as is now described. The method and
apparatus of the present invention permit the accurate sensing of the
height of a solder nozzle above a circuit board during operation of the
solder nozzle. A non-contact limit switch, which provides an indication of
the presence of a planar surface at a preselected proximity, is mounted in
a fixed relationship with a solder nozzle which is manipulatable by means
of a robotic arm. A reference surface is provided which includes a
proximity sensor, such as a through beam fiber optic switch. The proximity
sensor is mounted a predetermined distance above the reference surface and
is utilized to detect the solder nozzle as it is moved toward the
reference surface. By noting the position coordinates of the robotic arm
at which the proximity sensor indicates the presence of the solder nozzle
at the predetermined distance above the reference surface, and the
position of the robotic arm at which the non-contact limit switch closes
in response to the proximity of the reference surface, it is possible to
accurately calculate a calibration offset value which may be utilized in
conjunction with the output of the noncontact limit switch to precisely
position the solder nozzle at a desired distance above a circuit board. In
one preferred embodiment of the present invention, the solder nozzle is
repeatedly moved toward the reference surface during calibration to ensure
that the calibration offset value obtained is statistically significant.
BRIEF DESCRIPTION OF THE DRAWING
The novel features believed characteristic of the invention are set forth
in the appended claims. The invention itself however, as well as a
preferred mode of use, further objects and advantages thereof, will best
be understood by reference to the following detailed description of an
illustrative embodiment when read in conjunction with the accompanying
drawings, wherein:
FIG. 1 is a partially schematic, side view of the solder nozzle height
sensor system of the present invention; and
FIG. 2 is a partially schematic perspective view of a reference surface and
calibration jig which may be utilized in the solder nozzle height sensor
system of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
With reference now to the FIGS. and in particular with reference to FIG. 1,
there is depicted a partially schematic side view of the solder nozzle
height sensor system of the present invention. As is illustrated, a solder
deposition system 10 is depicted which includes a solder nozzle 12 and a
plurality of supply tubes 14 and 16. As those skilled in the art will
appreciate, supply tubes 14 and 16 may be utilized to supply inert gas or
flux, as well as solder into a heated plenum to form a molten mass of
solder. Solder nozzle 12 is then preferably moved over a plurality of
wettable contacts on the surface of a circuit board or other planar
member, such that a uniform amount of solder is deposited on each wettable
contact.
Solder nozzle 12 is preferably mounted to robotic arm 18, which is manually
or electronically manipulatable under the control of an operator or a
properly programmed robot control unit, such as robot control unit 32. As
is illustrated, robot control unit 32 preferably provides a plurality of
electronic signals which are coupled to various servos and motors which
are utilized to manipulate robotic arm 18. In this manner, solder nozzle
12 may be accurately and efficiently moved throughout a preprogrammed path
above the surface of a circuit board or other planar member. Of course,
variations in the thickness of a circuit board make it imperative that a
means be provided to accurately adjust the height of solder nozzle 12
above the surface of a circuit board, so that the amount of solder
deposited on the wettable contacts thereon is uniform and of desired
thickness.
In accordance with an important feature of the present invention, a rigid
bracket 20 is also mounted to robotic arm 18 in conjunction with solder
nozzle 12. Rigid bracket 20 is preferably formed of a metallic material or
other highly rigid substance and serves as a mounting point for mounting
member 22. As is illustrated, mounting member 22 is preferably mounted in
a downward direction from rigid bracket 20 and is generally oriented
parallel to the axis of solder nozzle 12.
At the lowermost end of mounting member 22 is a pneumatic switch nozzle 24.
Pneumatic switch nozzle 24 is utilized, in the depicted embodiment of the
present invention, as a non-contact limit switch and operates on
principles well known to those skilled in the art of such limit switches.
A flow of oxygen, nitrogen, or other suitable gas is preferably coupled to
pneumatic switch nozzle 24 by means of pneumatic supply lines 26 and 28
and is focused, utilizing nozzles or other techniques, onto the surface of
a planar member toward which solder nozzle 12 is manipulated.
As pneumatic switch nozzle 24 reaches a predetermined proximity to a planar
member, such as reference surface 38, variations in the pressure of the
gaseous material flowing through pneumatic switch nozzle 24 are detected
by limit switch 30 and may be utilized to accurately indicate the presence
or absence of a planar member within a preselected proximity.
Pneumatic switch nozzle 24 and its associated limit switch 30 may be
obtained commercially. One such product is marketed by the Nippon
Pneumatic/Fluidics Systems Company, Ltd., as Model NO. DAS-05-05. In the
depicted embodiment of the present invention, the preselected proximity
setting of pneumatic switch nozzle 24 is typically set to 0.040 inches.
This distance is detectable to within an accuracy of plus or minus 0.0015
inches, utilizing the device discussed above.
As is illustrated, the output of limit switch 30 is coupled to processor
34. Processor 34 may be any suitably programmed microprocessor based
device which may be utilized in conjunction with robot control unit 32 and
keyboard 36 to perform the rudimentary mathematical calculations necessary
to achieve a desired solder nozzle height.
Still referring to FIG. 1, a reference surface 38 is provided. Mounted
thereon is a calibration jig 40. Calibration jig 40 is preferably a
semi-circular or horseshoe shaped apparatus which encloses a gap, across
which is provided on optic beam 42. In a manner which will be explained in
greater detail herein, optic beam 42 is directed from one side of
calibration jig 40 to the other side and crosses the aperture therebetween
at a predetermined distance above the upper surface of reference surface
38.
Referring now to FIG. 2, there is depicted a partially schematic
perspective view of reference surface 38 and a calibration jig 40, which
may be utilized in the solder nozzle height sensor system of the present
invention. As is illustrated, optic beam 42 is preferably provided by
utilizing a plurality of fiber optic cables, such as fiber optical cable
44 and fiber optical cable 46. Apertures are drilled through calibration
jig end 50 and calibration jig end 52 of a sufficiently small diameter so
that the diameter of optic beam 42 is small enough to register the
presence of solder nozzle 12 within a desired degree of accuracy. In the
depicted embodiment of the present invention, the diameter of optical beam
42 is preferably 0.3 millimeters or smaller, leading to a capability of
detecting the presence of solder nozzle 12, within an accuracy of plus or
minus 0.0005".
As is illustrated, fiber optic cables 44 and 46 are both coupled to beam
switch 48. Beam switch 48 preferably includes a light source for
generating optic beam 42, which is coupled to one of the fiber optic
cables depicted. The presence or absence of the beam being returned by
means of the other fiber optic cable is then utilized to close or open a
solid-state switching device. The output of beam switch 48 is then
preferably coupled to processor 34 (see FIG. 1).
Referring now to both FIGS. 1 and 2, the calibration procedure which is
utilized with the solder nozzle height sensor system of the present
invention will be illustrated. First, solder deposition system 10 is
energized and solder nozzle 12 is brought up to operational temperatures.
Those skilled in the art will appreciate that it is important to raise
solder nozzle 12 to its actual operating temperature utilized during
solder deposition, so that errors induced by thermal expansion or
contraction of the material which forms solder nozzle 12 or the mounting
brackets therefore may be substantially reduced. Thereafter, robotic arm 8
is utilized to move solder nozzle 12 at a speed which is substantially
equal to the speed which will be utilized during actual solder deposition
technique.
Next, solder nozzle 12 and its associated pneumatic switch nozzle 24 are
moved downward toward the upper surface of reference surface 38. Robot
control unit 32, in conjunction with processor 34 and beam switch 48 are
utilized to stop the movement of solder nozzle 12 when optic beam 42 is
interrupted, indicating the presence of solder nozzle 12 at a
predetermined distance above the upper surface of reference surface 38.
The position of solder nozzle 12, as determined by robot control unit 32
is then stored.
Next, solder nozzle 12 is once again urged toward the upper surface of
reference surface 38, until such time as a variation in the pressure of
the gas flow being emitted from pneumatic switch nozzle 24 indicates that
the upper surface of reference surface 38 is within a preselected
proximity to pneumatic switch nozzle 24. At this point, limit switch 30
associated therewith closes, and sends a signal to processor 34, which is
utilized to stop the movement of robotic arm 18. The position of solder
nozzle 12, as determined by robot control unit 32 is then once again
recorded.
Next, in accordance with an important feature of the present invention,
these two preceding steps are repeated a number of times and each new
value for the position of solder nozzle 12 at the locations where optical
beam 42 has been interrupted and when pneumatic switch nozzle 24 indicates
the presence of the upper surface reference surface 38 at a predetermined
proximity are stored. Thereafter, these values are statistically averaged
and a standard deviation is calculated to permit the system to determined
whether or not the average values obtained are statistically significant.
Finally, a calibration offset value, which may be utilized to automatically
measure the offset from solder nozzle 12 to pneumatic electric nozzle 24,
is calculated. This calibration offset value is, quite simply, the average
position of solder nozzle 12 when optic beam 42 is interrupted, less the
position of solder nozzle 12 at the point when pneumatic switch nozzle 24
indicates the presence of the upper surface of reference surface 38 at a
predetermined proximity, less the predetermined height of optical beam 42
above the upper surface of reference surface 38. This calibration offset
value is then stored and may be utilized to accurately determine the
height of solder nozzle 12, in a manner which will be described below.
The automatic positioning of solder nozzle 12 at a selected distance above
a surface to be soldered is now accomplished following a procedure which
will be discussed herein. Once again, after raising the temperature of
solder nozzle 12 to an operating level and setting the speed of solder
nozzle 12 to that which will be utilized during normal soldering
operations, solder nozzle 12 and pneumatic switch nozzle 24 are urged
toward the surface to be soldered. Robot control unit 32 is then utilized
to stop the downward motion of solder nozzle 12 when the variation in
pressure within pneumatic switch nozzle 24, as detected in limit switch
30, indicates that pneumatic switch nozzle 24 is within a predetermined
proximity to the upper surface of the circuit board to be soldered.
The location of solder nozzle 12 at this point, as sensed by robot control
unit 32, is recorded. Next, an operator enters a desired solder offset
height, via keyboard 36, and robot control unit 32 then adjusts the height
of solder nozzle 12 by calculating a location corresponding to the desired
height. This is accomplished by adding the robot location indicated when
pneumatic switch nozzle 24 indicates the presence of a circuit board at a
preselected proximity to the desired solder height offset and the offset
calibration value previously calculated.
In this manner, a desired solder nozzle offset from the upper surface of a
circuit board may be accurately and efficiently maintained without the
utilization of a contact switch. By providing a noncontact limit switch,
such as pneumatic switch nozzle 24 and, in accordance with the method and
apparatus of the present invention, accurately determining the calibration
offset value which exists between the height of solder nozzle 12 and the
height of pneumatic switch nozzle 24 it is possible to accurately position
solder nozzle 12. This calibration offset value may be rapidly and
efficiently calculated by processor 34 and thereafter utilized by robot
control unit 32 to automatically adjust the height of solder nozzle 12 to
a desired level.
In view of the fact that this technique utilizes non-contact switching, it
performs well in the highly volatile environment which generally exists
near a soldering nozzle. Attempts at utilizing contact sensors to
determine the height of a solder nozzle above a circuit board have
generally met with failure due to the fact that such sensors quickly
become contaminated in this hostile environment. By utilizing a
non-contact pneumatic-electric switch, such as that comprising pneumatic
switch nozzle 24 and limit switch 30, the system becomes self-cleaning in
that a constant, low flow rate of oxygen, air or nitrogen passes through
pneumatic switch nozzle 24, constantly cleaning the aperture. Testing with
this system has proven that defect rates may be lowered dramatically with
this technique.
While the invention has been particularly shown and described with
reference to a preferred embodiment, it will be understood by those
skilled in the art that various changes in form and detail may be made
therein without departing from the spirit and scope of the invention. For
example, this system will work well in any robotic system in which a drill
or liquid dispenser must be brought to within a precisely selected height
above a planar member which is contaminated by dust and/or corrosive
gasses which would render a contact sensor inaccurate.
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